In recent years, much research has been devoted to the development of fuel cell systems to generate energy for automotive and other applications. A fuel cell system produces electricity by harvesting electrons from hydrogen gas. Oxygen is reduced by the harvested electrons and combined with hydrogen protons to produce water as a by-product. Fuel cell vehicles are highly efficient and environmentally-friendly.
A typical conventional PEM (polymer electrolyte membrane) fuel cell system includes multiple fuel cells, each of which includes a polymer electrolyte membrane interposed between an anode catalyst layer and a cathode catalyst layer to form a membrane electrode assembly (MEA). A gas diffusion medium (GDM) layer engages each catalyst layer, and a bipolar plate engages each GDM layer. The anode side bipolar plate is provided with flowfield channels which distribute a reactant gas, which contains hydrogen rich gas or may be pure hydrogen gas, to the anode catalyst layer through the anode side GDM layer. The cathode side bipolar plate is likewise provided with flowfield channels which distribute an oxidant gas, which may be air and contains oxygen or may be pure oxygen, to and reactant water vapor away from the cathode catalyst layer through the cathode side GDM layer.
During operation of the fuel cell system, hydrogen gas is split into electrons and protons at the anode catalyst layer. The protons are passed from the anode catalyst layer, through the electrolyte membrane and to the cathode catalyst layer. The electrons are distributed as electrical current from the anode catalyst layer, through an external circuit to drive an electric load, and then to the cathode catalyst layer. At the cathode catalyst layer, molecular oxygen is split into oxygen atoms, which combine with the electrons and hydrogen protons to form water. The water is distributed from the fuel cell system through the flowfield plates of the cathode side bipolar plate. In the fuel cell system, multiple individual fuel cells are typically stacked in series to form a fuel cell stack in which voltages and quantities of electricity proportional to the number of fuel cells are generated.
In a typical PEM fuel cell system, fuel cell stack modules are constrained by a specification or predetermined fuel gas/oxidant gas delta pressure vs. time profile which maintains a certain hydrogen/air delta pressure vs. time value, defined as changes in the hydrogen pressure relative to the air pressure at the fuel cell stack inlet over time. Normally, a pressure control device in the fuel cell system monitors the hydrogen and air pressures at the stack inlet and supplies hydrogen and air to the stack in the proper hydrogen/air pressure gradient in such a manner that the hydrogen/air delta pressure vs. time value is constrained within the specification. However, as the pressure control device ages over time, the pressure regulation capability of the pressure control device typically degrades, and consequently, its ability to control hydrogen pressure in the anode loop of the stack, particularly at idle flowrates, decreases. If the hydrogen/air delta pressure vs. time value drifts beyond the specification, particularly by the introduction of excess quantities of hydrogen into the stack via the defective or degrading pressure control device, then the excess hydrogen may migrate from the anode side into the cathode side of the stack. A common solution to this problem is to purge the excess hydrogen from the anode side of the fuel cell stack to the atmosphere. However, this method adversely affects fuel economy and hydrogen emissions.
Accordingly, an anode loop pressure control method is needed in which excess hydrogen in an anode loop of a fuel cell stack in a fuel cell system is consumed (1) by increasing the current demand on the stack, (2) periodically shutting off the fuel supply to the stack and/or (3) raising the operating pressure of the system. This adaptation of the system operating characteristics eliminates excessive quantities of hydrogen from the anode loop of the stack, thus mitigating fuel cell degradation while conserving fuel economy and hydrogen emissions.